Godless Commie

iTrader: (2)

The car is a 1998 NB, 1.8 engine.

Hotside MP62, custom header (thanks Gibbp), custom exhaust all the way to the rear bumper. No cat.

COP ignition

Custom 130 Amp alternator.

Skunk2 big throttle body

No A/C

No P/S pump (p/s converted to electric)

FM Stage 1 clutch

Mazda "competition" engine mounts

No intake manifold brace

Blocked off EGR, no EGR tube.

Coolant reroute

"Built" engine with 9.00:1 JE pistons, M-Tuned rods, Supertech valvetrain


I will gladly provide more specs if I missed anything of importance.

The problem is a very pronounced engine vibration or buzz between 2550 and 2950(ish) rpm range.

This happens regardless of:
* whether the engine is in gear or just revved in neutral while parked.
* whether the engine is accelerating or decelerating
* whether or not the supercharger belt is installed
* whatever gear the car may be in.
* whatever temp the engine may be at
* load
This thing had been and is happening before and after all the modifications listed above. (Meaning, all of those items can be ruled out as the culprit)


It's not the gearbox or the diff, because it happens in gear, in neutral and with the clutch depressed in neutral or while in gear.

I can put my hand on the engine and really feel the high frequency vibration as I increase/decrease the rpm past that dreaded range.

Definitely not the heat shield. I have none at this time. (plus, the heat shield rattle is a symptom, not the cause when that's the case - something just rattles that sucker)

I have been searching the forums, but all I can find is bits and pieces of my symptoms. Never the whole thing in one case.

Any ideas?

rleete

iTrader: (21)

Well, rattles, buzzes and squeaks are indicators that something is loose. The smaller/thinner the part, the higher the pitch. So, I'd say it's a cover of some sort.

Godless Commie

iTrader: (2)

Actually, the entire engine vibrates in that specific rpm range.

And, there are no covers to rattle anymore. None. No heat shields, no exhaust manifold cover.

I have a one piece header/downpipe combo, and that's wrapped all the way to where the cat would be.

rleete

iTrader: (21)

Ya got me. Harmonics?

Godless Commie

iTrader: (2)

I am even thinking about running the engine with the crank pulleys, clutch and flywheel removed to eliminate the external factors.

Godless Commie

iTrader: (2)

Apparently.

How would I deal with harmonics, then?

I'm guessing I would have to remove the crank+rods+pistons and balance them in a specific way...

My pistons and rods were all within 1 gram of each other, so I did not bother with a finer balance at the time I built the engine.

rleete

iTrader: (21)

I have no clue. Maybe one of the engine builders would have some ideas.

BTMiata

iTrader: (5)

Subbed. I have basically an identical noise (I'm assuming it's the same but I'll have to get a video for comparison).

Same rpm's though and the description is just as I'd describe mine! My motor is stock but similar mods.. Reroute, egr delete, intake mani support delete, etc

DNMakinson

iTrader: (3)

I wouldn't run an engine w/out a flywheel, even if I could figure out how to crank it.

Schuyler

iTrader: (1)

Make sure you didn't test fit a strut tower brace and leave it resting on top of the valve cover disconnected from both sides. Not that people do that. And then drive about 3/4 of a mile. Today.

richyvrlimited

iTrader: (1)

I suffer from something similar, as does a mate.

The one thing that's similar, we all run/ran superchargers. I'm convinced it's a harmonic from the vibration dampner causing the issue. My vibration started when I installed the blower, and even after retuning to stock, the vibration stayed

I think the balancer became damaged in some way, possibly from the additional weight/stress of the additional 6 rib pulley? However my mate had a setup which used the OEM 4 rib belt. I guess the additional loads could also cause an issue?

sixshooter

iTrader: (12)

I am going to suggest checking the harmonic balancer. And you might pull it off and check the keyway for widening or other wear.

rleete

iTrader: (21)

Wasn't there a problem with some balancers breaking the damper (rubber?) internally?

BarbyCar

iTrader: (1)

I know you mentioned this was happening before mods but jic and assuming BRP type install, check the rear most exhaust extended stud where it comes through the blower bracket. I had the weld on the bracket fail there and ensuing similar vibration.

ymmv

Godless Commie

iTrader: (2)

This got me thinking...

And I pretty much solved it today.
It was the vibration damper / under brace for the blower...

Godless Commie

iTrader: (2)

Here's what I did:

This is half of the underbrace for the blower:
(I did not think to take a picture of the whole thing, and it's too late now - but you guys get the idea.)



This brace obviously has a vibration damper..

But a close inspection revealed this:





So, I not only replaced the bad damper, but modified the whole thing a little.

There is a long hex nut that connects the damper and the heim joint.
I went to a machine shop and had them make me one with r/h threads on one side, and l/h threads on the other.



I also bought a L/H thread heim joint for a couple bucks, and connected everything using lock nuts.
Installed it in place with the lock nuts loose, and started playing with the tension/neutral/compression setting of the brace.

The magic happened under slight compression.
90% of the vibration is gone.

The damper was the first one I saw at the industrial hardware store (under 2 bucks). (They were closing, so I had to hurry).

I may experiment with different diameters/densities/metal faced designs later when I have time, but this did it.

Just wanted to post details.. Could be of help for someone.

18psi

iTrader: (76)

hahah was JUST about to say brace too

had the same thing on my FFS when I only hand tightened the brace. car sounded like a duck at those RPM, it was awful

sixshooter

iTrader: (12)

You had an FFS? I thought their kits were supposed to be junk?

Braineack

iTrader: (62)

what's your throttle wire look like when you hit that rpm?

18psi

iTrader: (76)

yep
removed it literally 4 days after install